So, traditional steam generators are simple kettles with u-shaped tubes in them, this functions effectively as a co-current heat exchanger. (Babcock and Wilcox used OTSGs, but that appears to have died when they were bought by Westinghouse, and they also have issues with very small water inventories)
As such, the steam outlet temperature is normally approximately equal to the reactor inlet temperature. This represents a significant loss of exergy as the outlet temperature in a CANDU is 310C, but the steam temperature is only 260C - or in the case of an APWR it is 325C and 283C.

There is currently research ongoing in building compact steam generators using a PCHE - but there seem to be problems with flow stability, especially during various transients.
They don't appear to like phase changes.

So how about placing a counter-flow heat exchanger at the top of the steam generator, where the primary fluid could be contacted with the single phase flow that is the steam flow from the stop of the steam generators.
Heat Transfer Coefficient for a water/high pressure gas heater with a PCHE is 1-4kW/square metre-kelvin.
And 100 bar is low enough pressure to still get 1300 square metres per cubic metre.

Which translates to 3.25MW/cubic metre-kelvin.

If the pressure drop of a PCHE is too high you could use a Heatric hybrid heat exchanger, with the primary fluid in the etched channels and the fin formed channels containing the primary steam.

In the APWR (a four loop PWR) will produce 625kg/s of steam. With a thermal power of roughly ~1100MWt.
A superheater that increases the steam output temperature of ~100MWt.
Considering that the steam side inlet temperature of the heat exchanger is 283C but the reactor inlet temperature is ~288C, even with a one celsius outlet temperature difference the AMTD is 3C.
Which means that ~100MWt would require a superheater of only 10 cubic metres, assuming a PCHE and not a hybrid heat exchanger.
Considering the steam generator has a diameter of at least five metres at the top, we are looking at a vertical height addition of less than one metre even with the headers.

And having 324 celsius steam at 67 bar will get significantly better efficiency than at 283 celsius and the 41C of superheat will probably allow the high pressure turbines inlet seperators to be dispensed with.
And the principle is generalisable to any reactor that uses a traditional steam generator.

The pipe diameters of a conventional U - HX in a nuclear power plant are about 16mm ...a flow area of about 180mm2 per pipe.

1. You can adapt the manufacturing method of diffusion bonding but use bigger channels of 5 x 3 -5 mm giving a flow area of 15 - 25mm2. That means the HX is more compact as it is in todays HX and the pressure loss is in between today and PCHE.

2. You can use additive manufacturing methods to produce a honeycomb like HX. Please find my previous ideas attached. Disadvantage of these methods is the rough surface with higher pressure loss and higher corrosion risk.

Stability issues with conventional, herringbone type PCHE in two phase flow can be expected; pressure drop per meter is high and channels are small.

What you want is lower pressure drop per unit length, and either big channels or cross-connected channels.

Big channels have the obvious downside that it partly defeats the point of the PCHE.

However, there has been work on alternate PCHE channels; after all with printed circuit technology there's essentially no limit to what shape you can make - only a small difference in setup cost which is trivial for a big order like this.

Some of the most interesting work has been on the use of airfoil-type PCHE. This is an order of magnitude lower pressure drop/meter but with similar power density as herringbone. It's also essentially an open channel, with flow being able to travel straight or sideways. I suspect this will be a great help in terms of two phase flow stability.

This is why I am proposing to use a traditional steam generator to produce the steam and then only superheat it with a diffusion bonded exchanger.

And why I am considering a Dowtherm-A intermediate loop to heat the reheaters in my double reheat plant layout.
[And the double reheat with a relatively high first reheat pressure allows the moisture seperators to be dispensed with entirely, turbine exhaust according to my projections is superheated all the way down to 0.7 bar - so much so that special desuperheaters have to be fitted in the bleed lines for the feedwater heaters to avoid destroying so much exergy]

Yeah, i've been thinking alot about this to improve the efficiency of the power station plant.
[Even using powerformer-type Generators wound for 400kV because they have earth potential stator cores, allowing feedwater to be used directly for cooling]

I prefer open feedwater heaters because I prefer more feedwater pumps to some sort of let-down turbine for the high pressure feedwater heater drains.

But I have PCHEs in the reheaters, the superheater, the condenser circulating water cooler, in many of the auxiliary systems (turbine rotor hydrogen coolers) and other things.

As I understand it yes, you simply need a feedwater pump group (with redundancy) for each heater, which in the traditional calculation adds more complexity than can be justified.

However now nuclear boilers cost an enormous sum and pumps and drives for the same get relatively cheaper every year.

Additionally building pressure vessels large enough for high pressure open heaters becomes problematic - but I think prestressing largely solves that issue.

In my last flow sheet the condenser pressure was 3kPa, and the first three feedwater heaters were 7.4kPa, 25kPa and 70kPa, which means there is a total head of only 7m across them, so now that we have 72" blade turbines with exhaust diameters of over seven metres, we could potentially stack the first three heaters on top of each other and use the weight of the water in the heater 'hotwells' to provide the necessary pressure increase, whilst only requiring a single pump, the lowest pressure heaters could draw from the top of the turbine housing and the higher pressure heaters from the bottom fo the turbine housing, thus keeping the feed lengths short.

Idea is airfoil suction through the foil due to lift can reduce frontal drag (pressure drop?) of a heat exchanger, if you stuff the airfoil with a semi-dense carbon foam.

Current PCHE airfoil patterns use fairly small symmetric airfoils, but clever design could make alternating patterns of asymmetric airfoils. Not sure how one would fit the airfoils with carbon foam blocks though since they would be tiny. Maybe go larger?

That PDF also mentioned microencapsulated PCM spheres to run the heat transfer fluid at constant temp.

E Ireland wrote:As I understand it yes, you simply need a feedwater pump group (with redundancy) for each heater, which in the traditional calculation adds more complexity than can be justified.

However now nuclear boilers cost an enormous sum and pumps and drives for the same get relatively cheaper every year.

Additionally building pressure vessels large enough for high pressure open heaters becomes problematic - but I think prestressing largely solves that issue.

Building big pressure vessels without excessive wall thickness is pretty easy, using shipyard type steel plate - beam technology. Basically submarine tech, but easier as you only have to deal with internal pressure rather than much more limiting external pressure in a sub.

My question on open FW heaters all the way is more along the lines of can it work at all, from several angles. Basically does it work for all the 6-9 stages of FW heating, or is there some problem that prevents this (to my knowledge this is never done?)

In my last flow sheet the condenser pressure was 3kPa, and the first three feedwater heaters were 7.4kPa, 25kPa and 70kPa, which means there is a total head of only 7m across them, so now that we have 72" blade turbines with exhaust diameters of over seven metres, we could potentially stack the first three heaters on top of each other and use the weight of the water in the heater 'hotwells' to provide the necessary pressure increase, whilst only requiring a single pump, the lowest pressure heaters could draw from the top of the turbine housing and the higher pressure heaters from the bottom fo the turbine housing, thus keeping the feed lengths short.

That's pretty ambitious but sounds workable. Generally I'm not a big fan of stacking equipment due to access/maintenance/inspection/replacement reasons. I generally prefer larger floor plans with access from the top - 360 deg. access becomes unnecessary so a lot of space and complexity is saved in the end.

Steam injectors can have big applications, I think, in LWRs. A lot of the FW pump duty can be replaced by the steam injector and simultaneously allow open FW heaters. Two in one blow. I would not underestimate the issues with FW pump capacity - for the ESBWR for example, the FW pumps set the plant electrical (house) loads, as they have to be sized for transient water flow (some ATWS events IIRC) as well as having redundancy. These things are massive. Having lower pressure drop throughout the FW chain by using open FW heaters and/or airfoil PCHE would help a lot here, limiting house loads and improving efficiency.

Cyril R wrote:
My question on open FW heaters all the way is more along the lines of can it work at all, from several angles. Basically does it work for all the 6-9 stages of FW heating, or is there some problem that prevents this (to my knowledge this is never done?)

It is not common because of the pump requirements, but as far as I know there is no reason that it cannot be done.

Cyril R wrote:
That's pretty ambitious but sounds workable. Generally I'm not a big fan of stacking equipment due to access/maintenance/inspection/replacement reasons. I generally prefer larger floor plans with access from the top - 360 deg. access becomes unnecessary so a lot of space and complexity is saved in the end.

Yeah, I am trying to make sure equipment is either easy to reach or is unlikely to break down.
I have several centrifugal magnetically-coupled pumps in my feedwater train to circulate water through my dowtherm preheaters (water is circulated out of the bottom FWH hotwell, through the PCHE and then added to the spray flow), the idea being that they won't require huge amounts of maintenance.

Cyril R wrote:
Steam injectors can have big applications, I think, in LWRs. A lot of the FW pump duty can be replaced by the steam injector and simultaneously allow open FW heaters. Two in one blow. I would not underestimate the issues with FW pump capacity - for the ESBWR for example, the FW pumps set the plant electrical (house) loads, as they have to be sized for transient water flow (some ATWS events IIRC) as well as having redundancy. These things are massive. Having lower pressure drop throughout the FW chain by using open FW heaters and/or airfoil PCHE would help a lot here, limiting house loads and improving efficiency.

Been thinking about steam ejectors for feedwater heating, but apparently they have control issues at part load and starting up.
So I am thinking about sticking with water pumps for now.

Not sure but it might be feasible to have 2 or 3 steam injectors to cover the load range - like gears in a gearbox. Or dual turbo's to reduce turbo-lag. These injectors are small and simple so they wouldn't cost much, can have a lot of them. It adds a few valves but pumps need such valves too (check valves, isolation valves).

Also wondering if, since diffusion bonded heat exchangers are so short and and compact, whether we could put an annular shaped one inside the turbine casing (especially if the heating fluid is a liquid and can thus requires only small pipes), this would enable a reheat to be added without adding additional turbine cylinders.

I could then move to a relatively large number of reheaters without requiring a hugely complex and expensive mass of steam pipework.

Also pondering a full blown intermediate loop, with an ESBWR style isolation condenser to cool the Dowtherm in a Station Blackout Accident, with two loops to protect against an SB-secondary LOCA.

This would increase complexity significantly but the pressure drop in the primary heat exchanger would be smaller so total pumping power would be similar probably, and it would allow the adding of horizontal tube steam generators directly adjacent to the high pressure turbine inlets, with reduction in steam pressure drops and the easy accessibilty of the plant for maintenance (due to very low radiation fields in the steam generators) offseting the extra complexity.

It also excludes a main steam line break accident, and a CANDU-6 style kiloton range water spray will allow a small primary containment and also suppress fires in the case of a Dowtherm Leak.